Wordpress 3.8.3 3.9 upgrade issue.

Just recently moved a domain from another host to my newly setup Debian Wheezy nginx setup on a VPS.
For some reason I kept getting error 500 trying to load a page no matter what I did.
Checked the logs and saw

After trying all the suggested site moving fixes I could find, like changing the .htaccess file, disabling plugins & themes, etc. - it occured to me that I had already moved another site that was using Wordpress and it had worked without any issues.

Further investigation revealed the working site was using Wordpress 3.8.3. So I downgraded the non working site from 3.9 to 3.8.3 and it worked instantly.

So the question is - Why did 3.9 break?

PHP is on PHP Version 5.4.4-14+deb7u9
MySQL version is 5.5.37-0+wheezy1

Just to make it even stranger. I tired doing a new wordpress install with version 3.8.3 on a different site. I used the same files that somehow magically fixed the existing site. All I get is an error 500. And in the error log for the site ( /var/log/ispconfig/domain ) here is what I get.

It's just been so inconsistent so far on how it reacts to WP. like it worked fine moving over a WP 3.8.3 site, but not moving 3.9 using the same procedure. ( Import database, move files, update config )
And yesterday got errors trying to create a brand new 3.8.3 site with a blank database. Then just for grins I decided to try it with the 3.9 - and it worked!

I don't mind if there are things that will or will not work on my system as long as I know what they are, and why they won't work so I can explain that to customers.

I did at some point during this adventure restart both nginx and php5-fpm. But did not make any configuration changes to either.

Code:

service php5-fpm restart
service nginx restart

Scratching my head on this one a bit. For now I'm just going to ignore it and move on, I guess. It's a low volume server, 10 or so sites. Only 2 of which are running Wordpress and one of those is a personal site.